###############################################################################
# Build documentation
###############################################################################
option(BUILD_DOC "Build LAMMPS HTML documentation" OFF)

if(BUILD_DOC)
  # Current Sphinx versions require at least Python 3.8
  # use default (or custom) Python executable, if version is sufficient
  if(Python_VERSION VERSION_GREATER_EQUAL 3.8)
    set(Python3_EXECUTABLE ${Python_EXECUTABLE})
  endif()
  find_package(Python3 REQUIRED COMPONENTS Interpreter)
  if(Python3_VERSION VERSION_LESS 3.8)
    message(FATAL_ERROR "Python 3.8 and up is required to build the HTML documentation")
  endif()
  set(VIRTUALENV ${Python3_EXECUTABLE} -m venv)

  find_package(Doxygen 1.8.10 REQUIRED)
  file(GLOB DOC_SOURCES CONFIGURE_DEPENDS ${LAMMPS_DOC_DIR}/src/[^.]*.rst)

  add_custom_command(
    OUTPUT docenv
    COMMAND ${VIRTUALENV} docenv
  )

  set(DOCENV_BINARY_DIR ${CMAKE_BINARY_DIR}/docenv/bin)
  set(DOCENV_REQUIREMENTS_FILE ${LAMMPS_DOC_DIR}/utils/requirements.txt)

  set(SPHINX_CONFIG_DIR ${LAMMPS_DOC_DIR}/utils/sphinx-config)
  set(SPHINX_CONFIG_FILE_TEMPLATE ${SPHINX_CONFIG_DIR}/conf.py.in)
  set(SPHINX_STATIC_DIR  ${SPHINX_CONFIG_DIR}/_static)

  # configuration and static files are copied to binary dir to avoid collisions with parallel builds
  set(DOC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR}/doc)
  set(DOC_BUILD_CONFIG_FILE ${DOC_BUILD_DIR}/conf.py)
  set(DOC_BUILD_STATIC_DIR ${DOC_BUILD_DIR}/_static)
  set(DOXYGEN_BUILD_DIR ${DOC_BUILD_DIR}/doxygen)
  set(DOXYGEN_XML_DIR ${DOXYGEN_BUILD_DIR}/xml)

  # copy entire configuration folder to doc build directory
  # files in _static are automatically copied during sphinx-build, so no need to copy them individually
  file(COPY ${SPHINX_CONFIG_DIR}/ DESTINATION ${DOC_BUILD_DIR})

  # configure paths in conf.py, since relative paths change when file is copied
  configure_file(${SPHINX_CONFIG_FILE_TEMPLATE} ${DOC_BUILD_CONFIG_FILE})

  add_custom_command(
    OUTPUT ${DOC_BUILD_DIR}/requirements.txt
    DEPENDS docenv ${DOCENV_REQUIREMENTS_FILE}
    COMMAND ${CMAKE_COMMAND} -E copy ${DOCENV_REQUIREMENTS_FILE} ${DOC_BUILD_DIR}/requirements.txt
    COMMAND ${DOCENV_BINARY_DIR}/pip $ENV{PIP_OPTIONS} install --upgrade pip
    COMMAND ${DOCENV_BINARY_DIR}/pip $ENV{PIP_OPTIONS} install --upgrade ${LAMMPS_DOC_DIR}/utils/converters
    COMMAND ${DOCENV_BINARY_DIR}/pip $ENV{PIP_OPTIONS} install -r ${DOC_BUILD_DIR}/requirements.txt --upgrade
  )

  set(MATHJAX_URL "https://github.com/mathjax/MathJax/archive/3.1.3.tar.gz" CACHE STRING "URL for MathJax tarball")
  set(MATHJAX_MD5 "b81661c6e6ba06278e6ae37b30b0c492" CACHE STRING "MD5 checksum of MathJax tarball")
  mark_as_advanced(MATHJAX_URL)
  GetFallbackURL(MATHJAX_URL MATHJAX_FALLBACK)

  # download mathjax distribution and unpack to folder "mathjax"
  if(NOT EXISTS ${DOC_BUILD_STATIC_DIR}/mathjax/es5)
    if(EXISTS ${CMAKE_CURRENT_BINARY_DIR}/mathjax.tar.gz)
      file(MD5 ${CMAKE_CURRENT_BINARY_DIR}/mathjax.tar.gz)
    endif()
    if(NOT "${DL_MD5}" STREQUAL "${MATHJAX_MD5}")
      file(DOWNLOAD ${MATHJAX_URL} "${CMAKE_CURRENT_BINARY_DIR}/mathjax.tar.gz" STATUS DL_STATUS SHOW_PROGRESS)
      file(MD5 ${CMAKE_CURRENT_BINARY_DIR}/mathjax.tar.gz DL_MD5)
      if((NOT DL_STATUS EQUAL 0) OR (NOT "${DL_MD5}" STREQUAL "${MATHJAX_MD5}"))
        message(WARNING "Download from primary URL ${MATHJAX_URL} failed\nTrying fallback URL ${MATHJAX_FALLBACK}")
        file(DOWNLOAD ${MATHJAX_FALLBACK} ${CMAKE_BINARY_DIR}/libpace.tar.gz EXPECTED_HASH MD5=${MATHJAX_MD5} SHOW_PROGRESS)
      endif()
    else()
      message(STATUS "Using already downloaded archive ${CMAKE_BINARY_DIR}/libpace.tar.gz")
    endif()
    execute_process(COMMAND ${CMAKE_COMMAND} -E tar xzf mathjax.tar.gz WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
    file(GLOB MATHJAX_VERSION_DIR CONFIGURE_DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/MathJax-*)
    execute_process(COMMAND ${CMAKE_COMMAND} -E rename ${MATHJAX_VERSION_DIR} ${DOC_BUILD_STATIC_DIR}/mathjax)
  endif()

  # set up doxygen and add targets to run it
  file(MAKE_DIRECTORY ${DOXYGEN_BUILD_DIR})
  file(COPY ${LAMMPS_DOC_DIR}/doxygen/lammps-logo.png DESTINATION ${DOXYGEN_BUILD_DIR}/lammps-logo.png)
  configure_file(${LAMMPS_DOC_DIR}/doxygen/Doxyfile.in ${DOXYGEN_BUILD_DIR}/Doxyfile)
  get_target_property(LAMMPS_SOURCES lammps SOURCES)
  add_custom_command(
    OUTPUT ${DOXYGEN_XML_DIR}/index.xml
    DEPENDS ${DOC_SOURCES} ${LAMMPS_SOURCES}
    COMMAND Doxygen::doxygen ${DOXYGEN_BUILD_DIR}/Doxyfile WORKING_DIRECTORY ${DOXYGEN_BUILD_DIR}
    COMMAND ${CMAKE_COMMAND} -E touch ${DOXYGEN_XML_DIR}/run.stamp
  )

  if(EXISTS ${DOXYGEN_XML_DIR}/run.stamp)
    set(SPHINX_EXTRA_OPTS "-E")
  else()
    set(SPHINX_EXTRA_OPTS "")
  endif()
  add_custom_command(
    OUTPUT html
    DEPENDS ${DOC_SOURCES} docenv ${DOC_BUILD_DIR}/requirements.txt ${DOXYGEN_XML_DIR}/index.xml ${BUILD_DOC_CONFIG_FILE}
    COMMAND ${DOCENV_BINARY_DIR}/sphinx-build ${SPHINX_EXTRA_OPTS} -b html -c ${DOC_BUILD_DIR} -d ${DOC_BUILD_DIR}/doctrees ${LAMMPS_DOC_DIR}/src ${DOC_BUILD_DIR}/html
    COMMAND ${CMAKE_COMMAND} -E create_symlink Manual.html ${DOC_BUILD_DIR}/html/index.html
    COMMAND ${CMAKE_COMMAND} -E copy_directory ${LAMMPS_DOC_DIR}/src/PDF ${DOC_BUILD_DIR}/html/PDF
    COMMAND ${CMAKE_COMMAND} -E remove -f ${DOXYGEN_XML_DIR}/run.stamp
  )

  add_custom_target(
    doc ALL
    DEPENDS html ${DOC_BUILD_STATIC_DIR}/mathjax/es5
    SOURCES ${LAMMPS_DOC_DIR}/utils/requirements.txt ${DOC_SOURCES}
  )

  install(DIRECTORY ${DOC_BUILD_DIR}/html DESTINATION ${CMAKE_INSTALL_DOCDIR})
endif()